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awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; c=0; next }
          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    data  { print data, desc, price; data="" }
' infile

including explanation:

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
            #set FS, OFS to comma; print "header" line

    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; c=0; next }
            #get the data if column#2 was non-empty and process next line

          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
            #cleanup unwanted strings

    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
            #collect the descriptions in 3 consecutive lines  

     data { print data, desc, price; data="" }
            #print the data, desc, price and reset the variables 
' infile 

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; c=0; next }
          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    data  { print data, desc, price; data="" }
' infile

including explanation:

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
     #this block will be executed only once before reading any line, and does: 
            #set FS (Field Separator), OFS (Output Field Separator) to a comma character
            #print the "header" line  ....

    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; c=0; next }
    #this blocks will be executed only when column#2 value was not empty, and does:
            #backup column#5 into "price" variable
            #also backup columns#1~4 into "data" variable
            #reset the "desc" variable and also counter variable "c"
            #then read next line and skip processing the rest of the code 

          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
            #this block runs for every line and replace strings above with empty string

    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    #this block runs at most 3reps and  
           #joining the descriptions in column#4 of every line
           #and read the next line until counter var "c" has value <=3 

     data { print data, desc, price; data="" }
     #if "data" variable has containing any data, then
           #print the data, desc, price and empty "data" variable 
' infile 

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; next }
          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    data  { print data, desc, price; desc=""; c=0 }
' infile

including explanation:

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
            #set FS, OFS to comma; print "header" line

    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; next }
            #get the data if column#2 was non-empty and process next line

          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
            #cleanup unwanted strings

    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
            #collect the descriptions in 3 consecutive lines  

     data { print data, desc, price; desc=""; c=0 }
            #print the data, desc, price and reset the variables 
' infile 

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; c=0; next }
          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    data  { print data, desc, price; data="" }
' infile

including explanation:

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
            #set FS, OFS to comma; print "header" line

    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; c=0; next }
            #get the data if column#2 was non-empty and process next line

          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
            #cleanup unwanted strings

    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
            #collect the descriptions in 3 consecutive lines  

     data { print data, desc, price; data="" }
            #print the data, desc, price and reset the variables 
' infile 

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
    $2    { price=$5; data=$1 FS $2 FS $3 FS $4; next }
          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    data  { print data, desc, price; desc=""; c=0 }
' infile

including explanation:

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
            #set FS, OFS to comma; print "header" line

    $2    { price=$5; data=$1 FS $2 FS $3 FS $4; next }
            #get the data if column#2 was non-empty and process next line

          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
            #cleanup unwanted strings

    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
            #collect the descriptions in 3 consecutive lines  

     data { print data, desc, price; desc=""; c=0 }
            #print the data, desc, price and reset the variables 
' infile 

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; next }
          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
    data  { print data, desc, price; desc=""; c=0 }
' infile

including explanation:

awk 'BEGIN{ FS=OFS=","; print "page,item,id,name,size,weight,glass,price" }
            #set FS, OFS to comma; print "header" line

    $2!=""{ price=$5; data=$1 FS $2 FS $3 FS $4; desc=""; next }
            #get the data if column#2 was non-empty and process next line

          { gsub(/ ?(mm \(size\)|Weight:|kg|Glass:) ?/, "") }
            #cleanup unwanted strings

    ++c<=3{ desc=(desc==""?"":desc OFS) $4; next }
            #collect the descriptions in 3 consecutive lines  

     data { print data, desc, price; desc=""; c=0 }
            #print the data, desc, price and reset the variables 
' infile